Our aging population is at an exceptionally high risk of debilitating falls, contributing significantly to reduced independence and quality of life. Despite conventional diagnostic and rehabilitative efforts, one-third of people over age 65 fall annually and 20-30% of these falls lead to moderate to severe injury. Remarkably, evidence even suggests that the rate of injurious falls among older adults is accelerating. Through an innovative sensorimotor paradigm using optical flow perturbations in a custom virtual environment, this proposal seeks to address the critical and immediate need for transformative new approaches for identifying and mitigating falls risk in our aging population. Our overarching goal is to investigate the efficacy of optical flow perturbations, particularly when applied during walking, to: (i) elucidate aging and falls history effects on standing and walking balance control, and (ii) subsequently condition successful balance control strategies through training. The first aim will tightly integrate virtual reality, visuomotor entrainment (i.e., the instinctive synchronization of motor responses to visual stimuli), and a series of clinical and self-reported benchmarks to investigate aging and falls history effects on the response to optical flow perturbations during standing and walking. We will test the hypothesis that optical flow perturbations during walking will distinguish age and falls history more effectively than those during standing and with effect sizes larger than those from conventional balance testing. The second aim will investigate sensory, motor, and cognitive-motor mechanisms governing susceptibility to optical flow perturbations. Using a strategically-selected combination of outcome measures and multivariate modeling, we will test the hypothesis that entrainment to optical flow perturbations will correlate most strongly with visual dependence and decreased somatosensory function, alluding to an age-associated process of multi-sensory reweighting that will emerge most prominently in walking. Finally, our third aim is designed to gain preliminary insight into the efficacy of prolonged optical flow perturbations to condition strategies used to successfully control walking balance in older adult fallers. In a randomized cross-over design, older adults with a history of falls will complete two treadmill training sessions ? one with (i.e., ?training? session) and one without (i.e., ?control? session) dynamic optical flow perturbations. We will test the hypothesis that older adults with a history of falls will adapt to prolonged exposure to perturbations, conditioning their step to step adjustments in walking balance control and improving their response to unexpected balance challenges following training. This research represents an interdisciplinary collaboration involving experienced investigators in biomedical engineering, physical therapy, and motor control. Successful completion of this R21 will provide the necessary mechanistic and preliminary efficacy information needed to design larger diagnostic and intervention studies to determine the value and applicability of perturbed optical flow to mitigate fall risk. With the advent of wearable and low cost virtual reality technology, this proposal is both timely and clinically feasible.